Re: [gdbserver][WinCE][ARM] gdbserver crash: issue with GetThreadContext???

[Pedro Alves <pedro at codesourcery dot com>]

On Friday 11 February 2011 10:59:29, Matthieu H wrote:

> /* Fetch register(s) from the current thread context.  */
> static void
> child_fetch_inferior_registers (struct regcache *regcache, int r)
> {
>   int regno;
> - win32_thread_info *th = thread_rec (current_inferior_ptid (), TRUE);
> + win32_thread_info *th = thread_rec (current_inferior_ptid (), FALSE);
> 
> This tells gdbserver not to load the context of the thread (=it does not call 
> GetThreadContext() so "current_inferior" is not corrupted)
> It's a dirty workaround cause I believe the thread context is required when 
> debugging!
> But at least I can connect using gdb on my host PC.
> The program crash later anyway, I believe the workaround is not good :-)
> 
> Can you please help me?
> Any idea what could be wrong?
> 
> GetThreadContext returns value "TRUE", meaning function succeed... maybe 
> arguments are wrong... but I'm not expert in gdb enough to say that.

Maybe the CONTEXT structure definition in mingw32ce's headers is
wrong for this device, and GetThreadContext is thus overwritting
memory it shouldn't?  There are a few simple ways to confirm that.
Try passing GetThreadContext a big buffer, memset to something
distinct, like 0xaa, and check how many bytes does that
change.  Compare to sizeof (CONTEXT).  

  char buffer[2048];
  memset (buffer, 0xaa, sizeof buffer);
  GetThreadContext (th->h, (CONTEXT *)buffer);
  // add code that dumps the buffer contents, or counts
  // how many bytes were changed.

Another simple check is to pad the end of win32_thread_info structure
with extra space.  Notice that the `context' field is the last of
the structure:

/* Thread information structure used to track extra information about
   each thread.  */
typedef struct win32_thread_info
{
  /* The Win32 thread identifier.  */
  DWORD tid;

  /* The handle to the thread.  */
  HANDLE h;

  /* Thread Information Block address.  */
  CORE_ADDR thread_local_base;

  /* Non zero if SuspendThread was called on this thread.  */
  int suspended;

#ifdef _WIN32_WCE
  /* The context as retrieved right after suspending the thread. */
  CONTEXT base_context;
#endif

  /* The context of the thread, including any manipulations.  */
  CONTEXT context;

<<<<<<<<<<<  add `char dummybuffer[1024];' or some such here.

} win32_thread_info;

If the theory is correct, then GetThreadContext will
clobber the dummy buffer instead of some other random thing.

-- 
Pedro Alves